The present invention generally pertains to frequency-dividing transponders and is particularly directed to an improved transponder for use in a presence detection system.
A presence detection system utilizing a frequency divider as a transponder is described in United Kingdom Patent Application No. 2,017,454. Such system includes a transmitter for transmitting a scanning signal at a first frequency in a surveillance zone; a transponder including an active frequency divider for detecting electromagnetic radiation at the first frequency and for transmitting a presence signal in response thereto at a second frequency to thereby detect the presence of the transponder in the surveillance zone. The electronic tags that the transponders are contained within are attached to articles of which detection is desired for enabling detection of the presence of such articles in the surveillance zone. Such presence detection systems are useful for detecting shoplifting, as well as for other applications.
A few examples of such other applications include detecting the presence of a person or vehicle carrying a transponder in a surveillance zone; detecting the presence of articles bearing transponders within a surveillance zone along an assembly line; and detecting the presence of keys attached to transponders in a surveillance zone at the exit of an area from which such keys are not to be removed.
The transponder is encased in a small tag that can be attached to an article in such a manner that it cannot be removed from the article without a special tool. When used in a shoplifting detection system, a sales clerk uses a special tool to remove the tag from the merchandise that is paid for; and the surveillance zone is located near the doorway for enabling detection of articles from which the tags have not been removed.
The transponder described in the aforementioned patent application includes a complex frequency divider that must be powered by an expensive long-life miniature battery.
A frequency divider that may be operated without a battery or any external power supply and is suited for use as a transponder in a presence detection system is described in U.S. Pat. No. 4,481,428. Such frequency divider includes a first circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; a second circuit that is resonant at a second frequency that is a subharmonic of the first frequency for transmitting electromagnetic radiation at the second frequency; and a semiconductor switching device having gain coupling the first and second circuits for causing the second circuit to transmit electromagnetic radiation at the second frequency solely in response to unrectified energy at the first frequency provided in the first circuit upon receipt of electromagnetic radiation at the first frequency.
Nonlinearities such as those due to saturation and hysterisis in the magnetization curves of thin magnetic alloy ribbons, including permalloy and untreated Metglas, are well-suited for producing higher-order harmonics, and this effect is described in French Pat. No. 763,681 to Picard and U.S. Pat. No. 4,298,862 to Gregor, et.al. It is not believed possible, however, to be able to create subharmonics and frequency-division behavior with such ribbons by utilizing only the inherant nonlinearities of their magnetization curves.